The present application relates to the examination of objects using different image acquisition modalities. It finds particular application to the use of ultrasound and x-rays in mammography examinations. It also relates to medical and other applications where information from multiple imaging modalities can be used to provide additional information about the structure and/or function of an object.
X-ray devices, in general, generate one or more 2-D images of an object under examination. The object is exposed to radiation, and an image is formed based upon the radiation absorbed by the object, or rather an amount of radiation that is able to pass through the object. Highly dense objects absorb more radiation than less dense objects, and thus an object having a higher density, such as a bone or mass, for example, will be apparent when surrounded by less dense objects, such as fat tissue or muscle.
In medical systems, x-ray devices are commonly used to detect broken bones, masses, calcium deposits, etc. that are not visible to the naked eye. One type of x-ray device is a mammography unit that generally comprises an x-ray tube, two compression paddles, and a detector array. The detector array and one compression paddle are mounted on a diametrically opposing side of the breast tissue (e.g., the object under examination) from the x-ray tube and the second compression paddle. The x-ray tube emits x-rays, and the x-rays traverse the breast tissue, while it is compressed between the two paddles. X-rays that traverse the breast tissue are detected by the detector array. In digital radiology, digital detectors (of the detector array) detect the x-rays, and reconstruction algorithms are used to create one or more two-dimensional (2-D) images of the breast tissue in the latitudinal dimension (e.g., orthogonal to a center x-ray beam and/or parallel to the detector array).
While 2-D x-ray images are useful in mammography and other applications, these images provide little or no resolution in the longitudinal direction (e.g., parallel to the x-ray beam and/or orthogonal to the detector plane formed by the detectors). On a breast examination, for example, a 2-D image cannot provide information about whether a mass is nearer the x-ray tube or the detector array. A less dense, but potentially cancerous mass, for example, may be masked by a more dense target, such as scar tissue, if the mass and scar tissue have a similar latitudinal coordinate (e.g., one target is on top of the other). Additionally, many (e.g., 85 percent in breast cancer screenings) positive findings are false positives (e.g., are not related to breast cancer). Therefore, patients are ordinarily called back for further testing if a positive finding is detected.
Ultrasound imaging is one common method used to confirm or reject an initial positive finding. Typically, an ultrasound probe transmits high-frequency sound waves (e.g., pulses) into the object under examination. As the sound waves travel through the object, some of the sound waves interact with a more dense target (e.g., mass, scar tissue, etc.), for example, that reflects a larger number of sound waves and/or causes a more significant attenuation of the sound waves (relative to less dense targets within the object). The sound waves that are reflected (e.g., echoes) are detected by the probe, and an ultrasound device calculates the distance from the probe to the more dense object and/or the intensity of the echoes. An image of the target inside the breast is formed based upon the calculations.
While current cancer screening techniques have proven effective for detecting early signs of cancer in some situations, there remains room for improvement. The x-ray scanning and ultrasound imaging are typically done at different times and in different physical positions. For example, in breast cancer screening, the mammography exam is usually done with a woman standing up and the breast tissue in a compressed state, while the ultrasound exam is done with the woman flat on her back and the breast stretched out (e.g., to reduce the distance the sound wave has to travel in the breast, thereby improving the image quality). Therefore, it is difficult to compare the images and detect similar details in the x-ray and the ultrasound images. Additionally, initial false positives can generate feelings of anxiety or distress that can last well after the ultrasound confirms that the initial positive finding was false.